.1863.] DR. J. S. BOWERBANK ON THE SPONGILLID^. 4G3 



membrane without any approximation to order, and have an average 

 length of Y^u ii^ch, and are ^^ inch in greatest diameter. They 

 vary to some extent in their dimensions ; but their form is always 

 fusiformi-acerate, the spines are abundant, conical, and acutely ter- 

 minated at all parts of the spiculum, but are not very strongly 

 produced. The interstitial membranes are also plentifully supplied 

 with the same description of tension-spicula as those of the dermal 

 membrane. The dermal membrane of S.Jluviatilis is aspiculous, 

 and in this character therefore we possess an organic difference in 

 the structure of the parts which leads us at once to a definite and 

 correct mode of determining the species, however closely they may 

 simulate each other iu form. 



The skeleton-spicula also differ in form from those of S.Jluviatilis: 

 in the latter they are purely acerate — that is, having the same dia- 

 meter throughout the greater portion of the shaft of the spiculum, 

 and attenuating only towards the terminations ; while iu the former 

 the spicula are stouter and shorter in their proportions, and the at- 

 tenuations commence at or very near the middle of the shaft of the 

 spiculum, and are therefore fusiformi-acerate in shape. 



But the greatest organic difference between this species and 5. 

 Jluviatilis exists in the spicula of the ovaries. In the latter the case 

 of that organ is strengthened and supported by a number of birotu- 

 late spicula, the rotulse supporting the inner and outer surfaces of 

 the case of the ovary, the shafts of the spicula being disposed at 

 right angles to the surfaces ; while in 8. lacustris the walls of the 

 ovary are totally destitute of birotulate spicula, but in their place 

 we find a considerable number of curved, acerate, spinous spicula, 

 not disposed at right angles to the surface of the gemmule, but im- 

 bedded in and lying parallel to the surface of that organ, thus afford- 

 ing a specific distinction so strikingly different from the correspond- 

 ing structures in S.Jluviatilis as to render the discrimination of the 

 species easy and certain whenever the ovaries are present. 



This species occurs plentifully at the bottom of the West Country 

 Timber-dock, on the south side of the Thames, near Rotherhithe. 

 It may frequently be found attached to the lower part of the large 

 mooring-posts near the central parts of the docks, about 8 or 10 feet 

 deep. I have never found it in this locality in shallow water or 

 near the surface like S. Jluviatilis, which is also abundant in the 

 same dock attached to the floating timber. On the contrary, at 

 Cookham, a few miles beyond Maidenhead, this species is abundant 

 on the posts and sides of the wharfing-boards ; and here it is always 

 found near the surface of the water, and has a very fine emerald- 

 green colour. 



Spongilla alba. Carter. 



Sponge sessile, encrusting, massive or subramose ; surface rugged. 

 Oscula simple, large, congregated iu groups, confluent. Pores in- 

 conspicuous. Dermal membrane thin, pellucid, furnished with an 

 irregular network of minute, slender, curved, entirely spined, acerate 

 spicula ; spines obtuse. Interstitial membranes abundantly furnished 



